During the 1980s, the US, Japan, and South Korea had reached—what many observers describe as—a golden age of trilateral security cooperation. US President Ronald Reagan, Japanese Prime Minister Nakasone Yasuhiro, and South Korean President Chun Doo-hwan not only shared their strategic goals but also built personal friendship. Such an intimate relationship—often shorthanded as “Ron-Yasu”—helped the three countries transform common priorities into policy momentum during the new Cold War era; the relationship offered a unified focus on the common enemy, the Soviet Union, and showcased a clear operational stage in the Western Pacific region. In contrast, the strategic environment of 2025 is far more complex. Yet from the structural point of view, such peculiarity is creating a strong foundation for deeper trilateral cooperation.

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Today’s US, Japan, and South Korea are facing a gradually consolidating Chinese-Russian partnership, a rapidly increasing North Korean nuclear threat, and the growing probability of a military conflict in the Taiwan Strait. As the Soviet Union’s challenge in the 1980s had coerced the trilateral countries to strengthen their alignment, the current threat dynamics—more multidimensional, networked, and geographically fluid—are similarly demanding consistent cooperation based on shared threat perception among Washington, Tokyo, and Seoul.

Three factors contributed to making trilateral coordination in the 1980s productive. First, the security environment offered strategic clarity. Soviet arms escalation in the Far East region, the Afghanistan war, and the deployment of medium-range nuclear missiles in Europe reinforced the common purpose of the democracies. Japan’s role as a shield and South Korea as a spearhead supplemented the US global strategy. Although conflictual elements occasionally surfaced in the trade sector—automobile, semiconductor, and dollar–yen imbalance issues—such economic conflicts were relatively separated from security affairs. Second, trust among the leaders functioned as a strong catalyst. Personal intimacy between Reagan and Nakasone reflected ideologically common beliefs such as anti-communism, trust among allies, and deterrence through strength. Irrespective of his domestic political issues, Chun Doo-hwan practically aligned with the US strategy and proactively participated in joint military exercises like Team Spirit. Such convergence reduced bureaucratic friction while showcasing alliance resolve against adversaries. Third, institutional habits strengthened the framework for trilateral cooperation. Under the summit-level cooperation, an intricate web of practical cooperation—consultation among staffs, intelligence exchange, joint military exercises, discussion of defense technology—was formulated. Such routine procedures provided some degree of resilience even when political or public pressure rose. This combination of clear threat perception, chemistry among leaders, and bureaucratic memory led to the success of the 1980s.

After four decades, the external environment is far riskier and comprehensive. Despite not being a formal military alliance, the partnership between China and Russia has reached an unprecedented level thanks to networks of mechanisms........

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